Scale control with terpolymers containing styrene sulfonic acid

ABSTRACT

Inhibition of scale is obtained by adding to an aqueous medium 0.5 to 500 ppm of a copolymer containing at least one of each of the following three monomers: 
     (a) monounsaturated carboxylic acids as well as salts and anhydrides thereof, the acids containing 3 to 5 carbon atoms, such as acrylic acid, methacrylic acid, maleic acid or its anhydride; 
     (b) acrylamidoalkane sulfonic acids and salts thereof, such as 2-acrylamido-2-methylpropane sulfonic acid; and 
     (c) styrene sulfonic acid and its salts.

REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 939,333 filedDec. 8, 1986, and entitled "Scale Control With Copolymers ContainingAcrylamidoalkane Sulfonic Acid".

BACKGROUND OF THE INVENTION

Most industrial waters contain alkaline earth and transition metalcations such as calcium, barium, iron, zinc, manganese, and magnesium,and several anions such as bicarbonate, carbonate, sulfate, phosphate,silicate, and fluoride. When combinations of these anions and cationsare present in concentrations which exceed the solubility of theirreaction products, precipitates form until these product solubilityconcentrations are no longer exceeded. For example, when the ionicproduct of calcium and carbonate exceeds the solubility of calciumcarbonate, a solid phase of calcium carbonate will form.

Solubility product concentrations are exceeded for reasons such aspartial evaporation of the water phase, change in pH, pressure ortemperature, and the introduction of additional ions which forminsoluble compounds with the ions already present in the solution.

As these reaction products precipitate on surfaces of the water-carryingsystem, they form scale or deposits. This accumulation preventseffective heat transfer, interferes with fluid flow, facilitatescorrosive processes, and harbors bacteria. This scale is an expensiveproblem in many industrial water systems, such as recirculating coolingwater systems in cooling towers, in that the scale causes delays andshutdowns for cleaning and removal of the scale.

Citric acid, gluconic acid, and other materials have been used in thepast to stabilize iron in solution and thus prevent its precipitation informs such as ferric hydroxide and ferric oxide, wherein iron is in thethird oxidation state. Citric acid and such materials effectivelymaintain iron in solution by forming complexes therewith which aresoluble in water and thereby, remain dissolved in water. Although citricacid and cognate materials are effective as stabilizing agents for ironin solution, they are not antiscalants and are ineffective against scalesuch as calcium carbonate, calcium phosphate, and calcium sulfate.

In U.S. Pat. No. 3,928,196, a method is disclosed for inhibiting scaleformation using two-component copolymers. The copolymers, which contain5-50 mole percent acrylamidoalkane sulfonic acid and 95-50 mole percentof acrylic acid or a similar acid, have molecular weight of 1,000 to10,000. The 25/75 mole percent copolymer of acrylamidomethylpropanesulfonic acid (AMPS) and acrylic acid (AA) is equivalent to 49/51 weightratio whereas the 75/25 mole percent copolymer of AMPS and AA, isequivalent to 89.5/10.5 weight ratio.

The Amick U.S. Pat. No. 4,711,725 discloses stabilization of an aqueoussystem by inhibiting precipitation of scale with a copolymer antiscalantcomprising 42 to 84% of (meth)acrylic acid and salts thereof, 11 to 40%acrylamidoalkane sulfonic acid, and 5 to 30% of one or more monomersselected from vinyl esters, vinyl acetate, and substituted acrylamide.The terpolymers of the Amick patent, when added to an aqueous system ina small amount, exhibit unexpectedly improved precipitation inhibitionand dispersion performance for phosphates, iron, zinc, and inorganicparticulates. The aqueous medium can contain or can be devoid of ironcontamination.

U.S. Pat. No. 4,707,271 discloses the use of copolymers of acrylic acidand a substituted acrylamide as antiscalants in presence of soluble ironin solution. Up to about 30 weight parts, per 100 weight parts of allpolymerized monomers, of other monomers can be polymerized with acrylicacid and substituted acrylamide.

This invention generally relates to inhibition of scale formation anddeposition in aqueous systems. This is accomplished by adding to anaqueous system at least a threshold inhibition amount of a copolymer ofat least three different monomers, i.e., monounsaturated carboxylicacid, salts and anhydrides thereof of 3 to 5 carbon atoms, anacrylamidoalkane sulfonic acid and salts thereof, and a styrene sulfonicacid. The resulting copolymer is soluble in water and is effective ininhibiting precipitation of scale, particularly calcium scale such ascalcium phosphate, calcium sulfate, and calcium carbonate. Inhibition ofscale precipitation is accomplished with the herein-defined copolymerswhich are effective against scale in the presence or the absence ofsoluble metal ions such as iron, manganese, and zinc. Soluble iron ionsreferred to herein exist in either the II or the III oxidation state.

Other related patent applications are concurrently filed for Messrs.Amjad and Masler including one entitled "Terpolymers For DispersingParticulates In An Aqueous Medium" and another entitled "Stabilizationof Metal Ions With Copolymers Containing Acrylamidoalkane SulfonicAcid". All of these cases disclose the use of same or similar polymers.

SUMMARY OF THE INVENTION

This invention is directed to the inhibition of precipitation ofscale-forming salts in an aqueous medium by addition thereto of aneffective threshold inhibition amount of a copolymer which contains atleast three different repeating groups. Such copolymers includepolymerized monounsaturated carboxylic acid of 3 to 5 carbon atoms andsalts thereof, polymerized acrylamidoalkane sulfonic acid, and a styrenesulfonic acid.

DETAILED DESCRIPTION OF THE INVENTION

Copolymers are described herein which are effective in reducingprecipitation of scale-forming salts in aqueous media. Such saltsinclude calcium phosphate, calcium sulfate, barium sulfate, magnesiumhydroxide, strontium sulfate, and calcium carbonate, and particularlycalcium salts such as calcium phosphate. Amount of such copolymer thatis added to an aqueous medium is at least an effective thresholdinhibition amount which is in the range of 0.5 to 500 parts per million(ppm) of the aqueous medium, preferably 1 to 50 ppm. Although theaqueous medium can be acidic, in a preferred embodiment, it is neutralor alkaline.

Specific applications contemplated for the copolymers disclosed hereininclude aqueous systems used in recirculating cooling towers, steamboilers, desalination, oil field applications in the secondary oilrecovery operations, flash distillation, as well as in aqueous systemssuch as sugar solutions. The copolymers are effective in reducingprecipitation of scale in the presence or the absence of soluble metalions such as iron, manganese, zinc, and mixtures thereof and mixturesthereof with other metal ions.

The copolymers can be in unneutralized or neutralized form. Suchcopolymers can be neutralized with a strong alkali, such as sodiumhydroxide, in which instance, the hydrogen of the carboxyl group in thecopolymer is replaced with sodium. With the use of an amine neutralizingagent, the hydrogen will be replaced with an ammonium group. Usefulcopolymers for purposes herein include copolymers that areunneutralized, partially neutralized, and completely neutralized.

The copolymers are soluble in water up to at least about 20% solidsconcentration, and they must, of course, be soluble in water in amountused, to be effective antiscalants. The copolymers contemplated hereinare polymers of at least three different monomers and include at leastone monomer selected from each one of the following groups (a), (b), and(c):

(a) monounsaturated carboxylic acids of 3 to 5 carbon atoms, salts andanhydrides thereof;

(b) acrylamidoalkane sulfonic acids and salts thereof containing up to6, preferably 1 to 4, carbon atoms in the alkane moiety; and

(c) a styrene sulfonic acid.

In addition to the above three requisite monomers, a small amount ofother or secondary copolymerizable monomers can also be used as long asthey do not substantially deleteriously affect performance of thecopolymers as antiscalants. Amount of such secondary copolymerizablemonomers can generally vary up to about 20% by weight, preferably up toabout 10%, and more preferably 2 to 10% by weight of the finalcopolymer. Such secondary copolymerizable monomers exclude substitutedacrylamides, vinyl esters and acrylate esters.

The copolymers suitable herein are random non-crosslinked polymerscontaining polymerized units of one or more of each of the monomers (a),(b), and (c), identified above, and can contain a small proportion ofpolymerized units of one or more of the secondary copolymerizablemonomers. The copolymers have weight average molecular weight of 1,000to 100,000, preferably 2,000 to 50,000, and more preferably 2,000 to20,000. The molecular weight given herein is measured by gel permeationchromatography.

The antiscalant copolymers disclosed herein contain 20 to 95% by weightof the polymerized carboxylic acid or its salt or anhydride, preferably40 to 90% and more preferably 50 to 70%; 1 to 60% by weight of thepolymerized sulfonic acid or its salt, preferably 10 to 50%; 5 to 50% ofa styrene sulfonic acid, preferably 5 to 40%; with up to about 20%,preferably up to 10%, of one or more of the secondary copolymerizablemonomers which do not deleteriously affect performance of the copolymersas antiscalants. The data herein shows performance for terpolymerscontaining about 40 to 70% acrylic acid, about 10 to 50% acrylamidomethylpropane sulfonic acid, and about 5 to 35% styrene sulfonic acid,with up to 15% of a secondary polymerizable comonomer.

The carboxylic acid monomers contemplated herein include monounsaturatedmonocarboxylic and dicarboxylic acids, salts and anhydrides thereof.Preferred in this class are monounsaturated monocarboxylic acids of 3 to4 carbon atoms and water soluble salts thereof, particularly acrylicacid and methacrylic acid. Because of its availability, effectivenessand low price, acrylic acid is particularly preferred. Repeating unitsof acrylic acid, methacrylic acid, and salts thereof are represented asfollows: ##STR1## where R is hydrogen or methyl and X can be hydrogen,alkali metal, alkaline earth metal, or ammonium, particularly hydrogen,sodium, potassium, calcium, ammonium, and magnesium.

The repeating units of acrylamidoalkane sulfonic acids and salts thereofare defined as follows: ##STR2## where R is hydrogen or methyl; X ishydrogen, ammonium, alkali metal or an alkaline earth metal,particularly hydrogen, ammonium or an alkali metal; and R¹ and R² areindividually selected from hydrogen and alkyl groups of 1 to 4 carbonatoms. In a preferred embodiment, R is hydrogen and R¹ and R² are eachan alkyl group of 1 to 3 carbon atoms. In this group of sulfonic acids,2-acrylamido-2-methylpropane sulfonic acid or AMPS® is a commercial,readily available monomer which is especially preferred for theantiscalant effectiveness described herein.

Repeating units of styrene sulfonic acids and salts thereof are definedas follows: ##STR3## where R is hydrogen or a lower alkyl group of 1 to6 carbon atoms, preferably hydrogen, and X is alkali metal or alkalineearth metal or ammonium, particularly hydrogen, ammonium or alkalimetal. A particularly suitable sulfonic acid is styrene sulfonic acidwhere R is hydrogen and the --SO₃ X group is at the 3 or 4 position onthe phenyl ring. The salts of styrene sulfonic acids are water-soluble.The sodium salt of styrene sulfonic acid is available commercially.

The monomers can be prepared, if desired, in a conventional manner butthey are commercially available and therefore, can be purchased.Polymerization of the monomers results in an essentially non-crosslinkedrandom copolymer, the molecular weight of which can be adjusted with alittle trial and error. The copolymer is preferably formed in a highyield ranging from about 50% to about 99% by weight of the comonomers.

It is also a requirement that the copolymer be soluble in water.Typically, the copolymer is used in water in the range of 0.5 to 500ppm. Thus, high solubility of water treatment compositions is notessential but desirable. The product is preferably shipped in drums as aconcentrated aqueous solution containing in the range of about 20% toabout 50% by weight of solids per 100 parts of solution, which requiressolubility to the extent of at least 20 weight parts per 100 parts ofwater.

Polymerization of the monomers identified herein can be carried out in amutual solvent for both, such as in a lower alkanol of about 1 to 6carbon atoms, or in water, with an effective amount of a free radicalinitiator sufficient to produce the desired composition within anacceptable period of time. The monomeric acids can be used as such orcan be in a partially or a completely neutralized form prior topolymerization.

The reaction is conveniently carried out in water as the only reactionmedium at a temperature in the range of about 30° to about 130° C.usually at atmospheric or slightly elevated pressure. The concentrationof the copolymer formed may range from about 5% to about 50% by weight,based on total solids, which solution can be shipped directly.

The copolymer may also be formed in an acyclic ketone, such as acetone,in an alkanol, in water, or mixtures thereof. If, for example, thecopolymer is formed in an organic solvent, or a mixture of an organicsolvent and water, the copolymer is converted from the organic solventsolution to a water solution. Typically, the organic solvent is strippedfrom the solution with steam or distilled off with subsequent additionsof water and repetition of distillation to remove the solvent, followedby the addition of water and a neutralizing agent such as causticsolution, ammonia, a hydrazine, or a low-boiling primary, secondary ortertiary aliphatic amine.

The final aqueous solution of polymer salt is preferably in the range ofabout pH 2 to about pH 8, with a total solids content of about 5 toabout 50% by weight of polymer in water.

The copolymers formed may have weight average molecular weight in therange of about 1,000 to about 100,000, preferably 2,000 to 50,000 andmore preferably about 2,000 to about 20,000, as determined by gelpermeation chromatography.

In a typical polymerization process, a glass lined or stainless steelreactor is charged with predetermined amounts of monomers along withsolvent and a free radical polymerization catalyst under a nitrogenblanket, and the reaction mixture allowed to exotherm under controlledtemperature conditions maintained by a heat-transfer fluid in the jacketof the reactor. The pressure under which the reaction occurs is notcritical, it being convenient to carry it out under atmosphericpressure.

The copolymers described herein in connection with threshold inhibitionof salt-forming scales can be used in combination with otherconventional additives wherever appropriate. Examples of some of theconventional additions include anti-precipitating agents, oxygenscavengers, sequestering agents, corrosion inhibitors, antifoamingagents, and the like.

The copolymers formed in the manner described herein, were used to treatwater which contained calcium ions and phosphate ions in a stirredpH-STAT test while maintaining constant pH and using an automatictritrator to gauge the effectiveness of the polymer for inhibiting theformation and deposition of calcium phosphate. The test was carried outas follows: a known volume of a phosphate salt solution, such as Na₂HPO₄, or another soluble phosphate salt solution, was transferred to aknown volume of distilled water in a double-walled glass cell to give afinal concentration of about 9 ppm of orthophosphate ions. To thissolution was added slowly and with continuous stirring a known volume oftesting polymer solution sufficient to give a dosage of 10 ppm.

A pair of glass and reference electrodes, which were calibrated beforeand after each experiment with standard buffer solutions of pH 7.00 and9.00, were then immersed in the solution which was maintained at 50° C.by circulating water through the outside of a glass cell jacket. Afterabout 45 minutes, a known volume of calcium chloride solution was slowlyadded to the continuously stirred solution containing phosphate andpolymer, to give a final calcium ion concentration of 140 ppm. The pH ofthe solution was then immediately brought to pH 8.50 by automaticaddition of 0.10 M NaOH solution. The pH of the solution was thenmaintained at 8.50±0.01 throughout the experiment using the pH-stattechnique. Solution samples were withdrawn after 22 hours, and analyzed,after filtration through 0.22 micrometer filter paper, fororthophosphate ions using the ascorbic acid method, as described indetail in "Standard Methods for the Examination of Water and WasteWater" 14th edition, prepared and published by American HealthAssociation. The instrument used for the colorimetric measurements was aBeckman 5270 Spectrophotometer.

Tests for calcium sulfate and calcium carbonate were carried out by themethod of Ralston, see J. Pet. Tech., Aug. 1969, 1029-1036.

The percent threshold inhibition (TI) attained for each experiment wasobtained using the following formula, shown in this case for calciumphosphate: ##EQU1## where (PO₄) exp=concentration of phosphate ion inthe filtrate in presence of the copolymer at time of 22 hours

(PO₄) final=concentration of phosphate ion in filtrate in absence of thecopolymer at time 22 hours

(PO₄ l ) initial=concentration of phosphate ion at time zero.

The invention disclosed herein is demonstrated by the following examplewhich shows threshold inhibition tests thereof and other relatedpolymers.

EXAMPLE 1

This example demonstrates threshold inhibition by various polymers,including the copolymers of this invention, of calcium phosphate scale.The tests were carried out in the manner described above for 22 or 24hours to determine percent threshold inhibition of calcium phosphate andcalcium sulfate, respectively, using the various polymers indicatedbelow with approximate weight average molecular weight. Results of thetests are given below in Table I.

                  TABLE I                                                         ______________________________________                                        Polymer   Monomer Wt.                                                                              Mol.   % Ca/P.sup.(a)                                                                        % CaSO.sub.4.sup.(b)                      Composition                                                                             Ratio      Wt.    Inhibition                                                                            Inhibition                                ______________________________________                                        AA:AMPS:  70:10:20    5,000 81      99                                        SSS       70:10:20   15,000 77      --                                                  60:10:30    5,000 93      97                                                  60:10:30   10,000 95      98                                                  60:10:30   15,000 89      99                                                  60:30:10    5,000 92(72)  97                                                  60:30:10   15,000 99(79)  --                                                  60:30:10   29,000 94(67)  --                                                  60:30:10   33,000 86(71)  --                                                  60:32.5:7.5                                                                              15,000 97      98                                                  50:30:20    5,000 92      94                                                  50:30:20   10,000 95      --                                                  50:30:20   15,000 96      97                                                  50:30:20   30,000 95      --                                                  40:30:30    5,000 66      96                                                  40:30:30   10,000 87      95                                                  40:30:30   15,000 95(85)  92                                                  40:50:10   15,000 83      71                                                  50:30:20   60,000 86      --                                                  50:40:10   50,000 95      --                                        AA:AMPS:  60:25:10:5 15,000 74      --                                        SSS:DADMAC                                                                    ______________________________________                                         .sup.(a) conditions: calcium = 140 ppm; phosphate = 9 ppm; pH = 8.50; T =     50° C.; time = 22 hr.; polymer = 10 ppm                                .sup.(b) conditions: calcium = 2000 ppm; sulfate = 4800 ppm; T =              66°  C.; time = 24 hr.; polymer = 4 ppm                           

The following contractions appear in the above table:

Ca/P--calcium phosphate

AA=acrylic acid

AMPS®=2-acrylamido-2-methylpropane sulfonic acid

SSS=sodium styrene sulfonate

DMDMAC=diallyldimethylammonium chloride

The numbers in parentheses indicate percent threshold inhibition ofcalcium phosphate in the presence of 1.0 ppm of soluble iron (III).

The sodium styrene sulfonate was commercial grade material obtained fromtwo different sources and is believed to be primarily the p-isomer.

It has been shown that the copolymers disclosed herein are effectiveantiscalants in the presence or absence of soluble metal ions. Typicalscales encompassed by the present invention include calcium scales suchas calcium phosphate, calcium sulfate, and calcium carbonate.

The copolymers described herein can be used in aqueous recirculatingsystems where the copolymers are effective in reducing deposition ofscale in the presence or the absence of soluble metals, such as iron. Asan open recirculating cooling system is brought into operation, theconcentration of scale-forming materials increases due to evaporation ofsome of the water. As the cycles of concentration increase, the scalingtendency of the system also increases. When an intolerable concentrationof scale-forming ions is reached, additional copolymer may be addedabove the normal dosage. However, in preferred operation mode, thesystem is operated at a steady state. Make-up water is added tocompensate for evaporation and other losses. A continuous orintermittent blowdown is used to remove some of the water containinghigh levels of scalants, and copolymer is added along with the make-upwater to maintain the desired copolymer concentration in the system. Inreverse osmosis systems, the system is operated at a steady state. Thedosage of polymer in the feed is chosen such that it will preventdeposition of scale in the concentrated brine solution. With appropriatechoice of polymer levels and operating conditions, a reverse osmosissystem may be operated for a very substantial length of time beforeshutdown must occur to clean the membranes. In such reverse osmosissystems, amount of soluble metals, such as soluble iron, is on the orderof up to about 10 ppm, preferably 1-5 ppm.

It has been shown that the terpolymers disclosed herein are effectivescale inhibitors in the presence or absence of soluble metal ions. Thisis surprising since one would expect that such terpolymers would losetheir effectiveness against scale in the presence of soluble metal ions.

We claim:
 1. A method of inhibiting precipitation of scale, includingcalcium phosphate, in an aqueous medium, in the presence or in theabsence of iron, comprising adding to said aqueous medium an effectiveamount of a water-soluble copolymer for the purpose of inhibitingprecipitation of said scale, said copolymer consisting essentially of(a) 40 to 70% by weight carboxylic monomer selected from acrylic acid,methacrylic acid, salts of such acids, and mixtures thereof, (b) 10 to50% by weight of a sulfonic monomer selected from2-acrylamido-2-methylpropane sulfonic acid,2-methacrylamido-2-methylpropane sulfonic acid, salts of said acids, andmixtures thereof, (c) 5 to 30% of a styrene sulfonic acid, and (d) up to20% by weight of one or more secondary copolymerizable monomer whichdoes not deleteriously affect performance of said copolymer asantiscalant, said secondary copolymerizable monomer excludingsubstituted acrylamides, vinyl esters, and vinyl acetate; said copolymerhaving a weight average molecular weight in the range of about 1,000 to100,000.
 2. Method of claim 1 wherein amount of said copolymer is 1 to50 ppm and its molecular weight is in the range of about 2,000 to50,000.
 3. Method of claim 2 wherein amount of said secondary monomer isup to 10% by weight and molecular weight of said copolymer is 2,000 to20,000.
 4. Method of claim 3 wherein said aqueous medium is alkaline andis selected from process water used in steam generating systems,recirculating cooling water systems, gas scrubbing systems, desalinationwater systems, and crude petroleum recovery systems.
 5. Method of claim4 wherein in said copolymer, amount of said styrene sulfonic acid isabout 5 to 30%.
 6. Method of claim 1 wherein said copolymer hasmolecular weight of about 2,000 to 50,000 and is selected from thefollowing copolymers, given in weight parts: (a) 40 to 70% acrylic acid,methacrylic acid, and mixtures thereof; (b) 10 to 50% acrylamidomethylpropane sulfonic acid; (c) 5 to 30% styrene sulfonic acid; and (d)up to 15% of one or more other polymerizable monomers.
 7. Method ofclaim 6 wherein said styrene sulfonic acids and salts thereof aredefined as follows: ##STR4## where R is selected from hydrogen and alkylgroups of to 6 carbon atoms, and X is selected from hydrogen, alkalimetals, alkaline earth metals, ammonium groups, and mixtures thereof. 8.Method of claim 7 wherein in the formula, R is selected from hydrogenand alkyl groups of 1 to 2 carbon atoms and X is sodium.
 9. Method ofclaim 7 wherein in the formula, R is hydrogen and the --SO₃ X group isat the 3 or 4 position on the phenyl ring.
 10. Method of claim 1 whereinsaid copolymer is selected from the following copolymers, given inweight parts:(a) 70/10/20 copolymer of AA/AMPS/SSS (b) 60/10/30copolymer of AA/AMPS/SSS (c) 60/30/10 copolymer of AA/AMPS/SSS (d)60/32.5/7.5 copolymer of AA/AMPS/SSS (e) 50/30/20 copolymer ofAA/AMPS/SSS (f) 40/30/30 copolymer of AA/AMPS/SSS (g) 40/50/10 copolymerof AA/AMPS/SSS (h) 50/40/10 copolymer of AA/AMPS/SSS (i) 60/25/10/5copolymer of AA/AMPS/SSS/DADMACwherein the contractions are defined asfollows: AA=acrylic acid SSS=sodium styrene sulfonateAMPS=2-acrylamido-2-methylpropane sulfonic acidDADMAC=diallyldimethylammonium chloride.